This invention relates generally to hydrocarbon pyrolysis, producing olefins, for example; and more particularly it concerns improvements in reaction tube configurations in such processes, leading to reduced coking.
In hydrocarbon pyrolysis, the primary products are typically olefins. They are favored by reactions with short hydrocarbon residence time, in the reactor, and low hydrocarbon partial pressure. To achieve these conditions, the reactor volume, and thus residence time, must be minimized, whereby reaction tubing is required. The reactor volume of tubular type is determined by its length and diameter.
In pyrolysis, there are two important considerations: the conversion of feedstock and the olefins selectivity. The extent of conversion measures the destruction i.e. reforming of the feedstock, and the olefins selectivity indicates the efficiency of the production of olefins from the destroyed feedstock.
The most efficient tubular reactor is a coil consisting of a single tube having small diameter, such a single tube reactor providing short residence time and low hydrocarbon partial pressure. Consequently, a high olefins selectivity is obtained. The disadvantage of a single tube reactor is that the capacity is low. A large number of coils is therfore needed for a given capacity of furnace, which makes the furnace more costly. In this regard, it is believed in the past that the flow in coil tubing in a convection heating section of the furnace should be slower than flow in tubing in a radiant heating section of the furnace. Such coils tend to "coke-up" in use, reducing their effectiveness, and olefin yield, and the larger the number of coils employed, the greater the coking problem due to changes in heating resulting from coil position in the furnace. A solution to these problems, prior to the present invention was not known.